Redox-Active, Boron-Based Ligands in Iron Complexes with Inverted

Research Article Cite This: ACS Catal. 2019, 9, 7300−7309

pubs.acs.org/acscatalysis

Redox-Active, Boron-Based Ligands in Iron Complexes with Inverted Hydride Reactivity in Dehydrogenation Catalysis Andreas Bäcker,† Yinwu Li,‡ Maximilian Fritz,† Maik Grätz,† Zhuofeng Ke,*,‡ and Robert Langer*,† †

Department of Chemistry, Philipps-Universität Marburg, Hans-Meerwein-Straße, 35032 Marburg, Germany School of Materials Science & Engineering, PCFM Lab, Sun Yat-sen University, Guangzhou 510275, China

‡

Downloaded via GUILFORD COLG on July 17, 2019 at 06:18:43 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: For a series of PBP-type iron(II) pincer complexes, the central donor group based on tricoordinate boron is demonstrated to be redox-active, formally yielding iron(0) and a boronium-species by reversible B−H reductive elimination. In contrast to common tricoordinate boron compounds, such as BF3, which are known to act as Lewis acid and Z-type ligand, the introduction of π-accepting phosphine substituents at the boron center leads to an umpolung of the bonding situation from R3B←Fe to L2RB→Fe in the reported complexes. The described iron(II) complexes are competent catalysts for the dehydrogenation of Me2NH-BH3. Depending on the substituents a homo- or hetertopic catalyst is formed. Experimental and quantum chemical investigations on the most active, homogeneous catalyst indicate that hydrogen liberation can proceed via different pathways, involving a hydrido ligand as the proton source or a carbanion bifunctional mechanism. The unprecedented catalytic mechanism and the unusual reactivity that allows for two-electron redox steps are attributed to the unique donor properties of the boron-based ligand. KEYWORDS: boron, iron, dehydrogenation, pincer ligand, metal ligand cooperation

1. INTRODUCTION Polydentate ligands containing functional groups based on boron are potential alternatives to well-established ligand platforms that have been used in transition metal complexes for bond activation reactions and in cooperative catalysis for decades.1,2 After numerous reports about highly electron rich boryl complexes and their application in bond activation,3−9 neutral moieties based on tricoordinate boron recently gained increasing attention as ligands, due to their flexible binding and their entirely different reactivity patterns.10−12 After the seminal work of Hill, Crossley, Owen and Parkin about metallaboratranes with a Lewis acidic BR3 moiety acting as an electron-accepting Z-type ligand,13−24 several polydentate ligands with a Lewis acidic boron center were developed over the past years.25−34 The ability to stabilize highly electron rich metal centers by flexible binding of a Z-type ligand was successfully exploited in the development of a novel type 3d metal (de)hydrogenation catalysts.30,32−35 In these complexes, the bound BR3-group can act as a cooperative site for E−H bond activation (E = H, SiR3), © XXXX American Chemical Society

serving as the hydride acceptor while the group E remains at the oxidized metal center. In addition to the classical boron-based Lewis acids such as BF3, it was recently shown that the isolation of a Lewis basic, tricoordinate boron compounds (II) is possible, if at least two neutral substituent with π-accepting properties (e.g., carbenes, carbonyl) are present.36−44 Subsequently, our group demonstrated that unstable L2BR-species (II) (e.g., L = PR3) can be stabilized in the coordination sphere of a transition metal.1,45,46 However, the reported PBP-type iron(II) complexes were obtained by an unusual rearrangement of a phosphine-borane complex47 and their formation is strictly limited to phenylsubstituents at the terminal phosphine groups and to CO and CN− as ancillary ligands,45,46 which in turn limits their application as catalyst in (de)hydrogenation reactions. Herein, we report a general synthetic route to a wide range of iron(II) Received: February 28, 2019 Revised: June 26, 2019 Published: June 27, 2019 7300

DOI: 10.1021/acscatal.9b00882 ACS Catal. 2019, 9, 7300−7309

Research Article

ACS Catalysis

complexes by the appearance of new triplet resonances for the terminal phosphine groups (Figure 2). The resulting cationic

complexes containing pincer ligands with a central (R3P)2HB: donor group. Our study shows that all iron complexes undergo a reversible B−H reductive elimination, but with different rates and different activation parameters. Finally, we demonstrate that the reported complexes are active catalysts for the dehydrogenation of Me2NH-BH3 and provide evidence that the iron-bound hydrido ligand rather serves as a proton source in the hydrogen liberation step, resulting in an reduced iron species. Although polarity-inverted activation and liberation of H2 was recently reported for a nickel(0) diphosphine complex with Z-type ligand based on boron,30,33 the acidic nature of certain metal hydrides has not yet been considered as key property for catalytic activity (Figure 1b). In view of the intensive efforts that have been

Figure 2. Reaction of boronium salts 1a−1c with [Fe(CO)5] (a: R = Ph, X = NH; b: R = Ph, X = CH2; c: R = Et, X = NH).50

complexes 3a−3c are obtained as pure products by precipitation or layering with Et2O. Complex 3a has been previously prepared by an unusual rearrangement of a phosphine-borane complex and the intramolecular B−H-oxidative addition of a boronium salt was proposed to be a key-step in this sequence, which is proofed now with the reported reactions.46 At low temperatures all three complexes give rise to triplet resonances between −9.3 and −10.8 ppm in the 1H NMR spectrum for the newly formed hydrido ligand (Table 1), while Table 1. Selected Structural and Spectroscopic Data of 3a−3c τ(ML5) (deg) dFe−B (Å) νCO (cm−1) δ(1HFe−H) (ppm) δ(31P) (ppm) δ(11B) (ppm)

Figure 1. (a) Conventional (left) and polarity-inverted (right) hydrogen liberation. (b) Lewis acidic tricoordinate boron compounds BR3 (I) and their mode of action as Z-type ligands in cooperative bond activation (top). Nucleophilic boron compounds L2BR (II), also known as ligand-stabilized borylenes, and the observed reactivity pattern reported herein (bottom).

3a

3b

3c

0.16 2.187(2) 1936, 1986 −9.59 54.3, 129.8 −25.0

0.05 2.226(7) 1924, 1970 −9.37 22.5, 78.3 −23.1

0.18−0.34 2.219(9)-2.216(8) 1912, 1958 −10.8 65.9, 138.4 −30.2

at ambient temperature the resonance corresponding to 3c is so broad that the 2JPH coupling is not resolved and the corresponding resonance in the 1H NMR spectrum of 3b even vanishes. The 31P{1H} NMR spectra show a triplet resonance for the terminal PPh2-groups and a broad resonance for the boron-bound phosphorus atoms, respectively. In line with previously reported complexes containing a ligand-stabilized borylene as a ligand,38,40,41 broad resonances between −23.1 and −30.2 ppm are observed in the 11B{1H} NMR spectrum. The dicarbonyl complexes 3a−3c give rise to two strong bands for C−O stretching vibrations between 1910 and 1990 cm−1, and the wavelength of these vibrations might be taken as qualitative estimate for the electron richness of the central iron atom. Based on this assumption, the metal center in 3c is more electron rich than in 3b and 3a. In the case of 1c the intermediate 2c can even be isolated. The NMR spectra of complex 2c suggest the coordination of an intact preligand 1c to a Fe(CO)3fragment. The identity of 2c and 3a−3c was finally confirmed by single crystal X-ray diffraction (Figure 3). An octahedral arrangement by a meridionally coordinated pincer ligand, two carbonyl and one hydrido ligand is observed for 3a−3c. The Fe−B bond distances are quite similar, ranging from 2.187 to 2.226 Å, which is significantly shorter than the those found for Z-type ligands in related iron complexes.51 In previous studies, we demonstrated by a combination of experimental and quantum chemical methods that (R 3 P) 2 BH-moieties are strong donor ligands.45,46,48,52,53 For 3a−3c further indication for the donor character of the central boron-based group in the pincer ligand can be derived from geometries of the penta-coordinated fragments without the boron-based ligand. An L-type ligand

devoted to catalytic dehydrocoupling reactions for the creation of heteronuclear bonds between p-block elements, our study provides an alternative mechanistic scenario that was not considered so far.

2. RESULTS AND DISCUSSION The preparation of iron pincer complexes with a neutral boronbased donor group was attempted by the oxidative addition of bisphosphinoboronium salts, adopting a recently developed method.48 For this reason we prepared a series of boronium salts by the reaction of BH2Br·SMe2 with two equivalents of the diphosphines 1,1-bis(diphenylphosphino)amine 1a (dppa), 1,1bis(diphenylphosphino)methane 1b (dppm) and 1,1-bis(diethylphosphino)amine 1c (depa), respectively. The new boronium salts 1b and 1c give rise to similar spectroscopic data as 1a and their identity was finally confirmed by single crystal Xray diffraction.49 Using this series of preligands, we addressed the question whether iron(0) complexes are able to react with the synthesized boronium salts under B−H-oxidative addition to the corresponding iron(II) pincer complexes. 2.1. Synthesis: Oxidative Addition of Boronium Salts. Treatment of [Fe(CO)5] with the boronium-salts 1a−1c in CH2Cl2 or tetrahydrofuran (THF) does not result in any reaction. However, upon irradiation with UV-light the colorless solution slowly turns orange and the 31P{1H} nuclear magnetic resonance (NMR) spectrum indicates the formation of new iron 7301


Research Article

ACS Catalysis

Figure 3. Molecular structure of the cations in 3b (a), 2c (b) and 3c (c) in the crystal lattice (counterions and carbon-bound hydrogen atoms are omitted for clarity; thermal ellipsoids are drawn at 30% (3b) or 50% (2c, 3c) probability).

requires a ”real” coordination site and will occupy the vertex of an octahedron formed with square pyramidal ML5-fragment. A Z-type ligand, as commonly assumed for BR3-ligands, would rather bridge an edge or a face of the polyhedron formed by the coordinated L- and X-type ligands. In consequence, a square pyramidal geometry of the ML5-fragment indicates the presence of a boron-based donor ligand and a trigonal bipyramidal arrangement points toward an acceptor ligand. Both geometries can be distinguished by the τ-parameter, which is 0 for square pyramidal and 1 for trigonal bipyramidal complexes. The τparameters between 0.05 and 0.34 for the ML5-fragments in 3a− 3c clearly indicate the presence of a donor ligand. A single crystal X-ray diffraction analysis of the isolable intermediate 2c revealed that the penta-coordinated iron(0) complex, in which the central iron atom is coordinated by three carbonyl ligands and the two terminal PPh2 groups in a distorted square pyramidal arrangement (τ = 0.26). Further irradiation with UV-light of complex 2c, dissolved in CH2Cl2, results in selective formation of 3c, which clearly shows that the reaction of the boronium salts 1a−1c with [Fe(CO)5] is a two-step process: The loss of two carbonyl ligands allows for coordination (i) of the cationic boronium salt and the dissociation of the third carbonyl ligand facilitates the B−H-oxidative addition (ii). 2.2. Redox-Reactivity: Reversible B−H Reductive Elimination. Complex 3a was previously demonstrated to undergo an intramolecular and reversible B−H reductive elimination in solution to give the penta-coordinated intermediate 4 with a coordinated bisphosphinoboronium group (Figure 4a). The line broadening of the resonance corresponding to the hydrido ligands in the 1H NMR spectra already indicated that dynamic processes for 3a are quite slow, whereas 3b and 3c are highly dynamic species. The 1H nuclear Overhauser effect spectroscopy (NOESY) NMR spectra of 3a− 3c at different temperatures reveal exchange correlations between the hydrido ligands and the boron-bound hydrogen atom of the central donor group, respectively. Based on kinetics, quantum chemical investigations and labeling experiments the exchange process in the deprotonated analogue of 3a was previously demonstrated to proceed via an intramolecular and reversible B−H reductive elimination that allows for the exchange of the coordinated B−H bond in 4a.46 In the case of complex 3c a second exchange process was detected at low temperature (213 K) in the 1H NOESY NMR spectra, in addition to the observable B−H reductive elimination in 3a−3c. Analytically pure samples of 3c give rise to a second set of resonances at 213 K of very low intensity in the 1H and 31P

Figure 4. (a) Redox-reactivity in solution of complexes 3a−3c. (b) cisand trans-isomers of 3c.

NMR spectra, which were assigned to the corresponding ciscomplex cis-3c (Figure 4b). A triplet resonance at −10.04 ppm (3JPH = 60.0 Hz) is observed for the hydrido ligand in the 1H NMR spectrum, which shows exchange correlations to the hydrido ligand of trans-3c in the 1H NOESY NMR spectrum at 213 K. This observation suggests the presence of a cis/transisomerization. To get further insights into the observed reactivity patterns, we estimated the overall exchange rate constants (k1) by line shape analysis of the 31P-decoupled resonances of the hydrido ligands at different temperature. The Eyring plot in Figure 5 shows a linear dependence of ln(k1/T) vs T−1 and allows for the extraction of activation parameters (Table 2). It becomes evident that in line with the strongest line broadening (coalescence at ambient temperature), for complex 3b the lowest barrier is found (ΔG‡ = 12.31 kcal·mol−1). Complex 3c showed significant broadening as well, which is reflected in a Gibbs enthalpy of activation of ΔG‡ = 13.93 kcal·mol−1. In contrast, complex 3a gives rise to well-resolved resonances in the 1 H NMR spectrum and an increased barrier for the overall exchange process (ΔG‡ = 16.18 kcal·mol−1), consisting of B−H reductive elimination, exchange of the coordinated B−H bond and B−H oxidative addition. A closer look on the activation parameters reveals that the entropy of activation outweighs the enthalpy term and results in the lowest barrier for complex 3b. The origin for the observed trend of barriers is not very easy to identify, as the applied ligand modifications from 3a to 3c affect both, the iron center and the central boron atom, and a likely 7302


Research Article

ACS Catalysis

two ancillary carbonyl ligands cause a reduced hydricity and a decreasing pKa value of the hydrido ligand in these complexes.54 Me2NH-BH3 was selected as the substrate for catalytic dehydrogenation reactions, due to the simplicity of different known reaction products and the possibility to gain mechanistic insights. In recent years, a number of iron-based catalysts was reported for the dehydrogenation of amine and ammonia boranes, ranging from simple iron phosphine complexes,55,56 such as [(Me3P)3Fe(H)(H2CPMe2)], to cyclopentadienyl complexes,57−59 pincer-type complexes with different kinds of ligands,60−62 monomeric63,64 and dimeric iron(I) complexes,65 complexes with β-diketiminate66 and P2N2-type ligands.67 Contrary to the reactivity toward unsaturated substrates, the dehydrogenation of Me2NH-BH3 was efficiently catalyzed by the PBP-type complexes 3a−3c (Table 3). The reaction with 2 mol % of precatalysts 3a−3c and 20 mol % of base cocatalyst (KOtBu) results in full conversion of dimethylamine borane (DMAB) at ambient temperature (23 °C) for all three precatalysts, 3a−3c, and the selective formation of (Me2NBH2)2 according to eq 1 (entries 1−3).

Figure 5. Eyring plot for the exchange rates obtained by line shape analysis.

[Fe]

2Me2NHBH3 ⎯⎯→ (Me2NBH 2)2 + 2H 2

Table 2. Activation Parameters for the Intramolecular B−H Reductive Elimination in 3a−3c Derived by Eyring Analylsis 3a 3b 3c

ΔH‡ (kcal·mol−1)

ΔS‡ (cal·mol−1·K−1)

ΔG‡ (kcal·mol−1)

6.64 ± 0.86 8.80 ± 0.26 8.44 ± 0.74

−32.00 ± 2.77 −11.76 ± 2.77 −18.14 ± 2.84

16.18 ± 1.67 12.31 ± 0.57 13.93 ± 1.58

(1)

Although the formation of the monomeric product Me2N-BH2 and the linear product Me2NH-BH2-NMe2-BH3 is not observed, the yield of the cyclic dimer (Me2N-BH2)2 differs for 3a−3c (62−83%) and different amounts of the dehydrogenation products (Me2N)2BH (9−14%) and (tBuO)2BH (7−20%) are observed by 11B{1H} NMR spectroscopy.68,69 The latter two compounds are likely to be formed by a second catalytic dehydrogenation reaction (eq 2), which can lead to full consumption of the employed base. The consumption of base might explain that yields are increasing, if the amount of base is increased from 1 to 10 equiv.

transition state involves a proton transfer from the iron center to the (R3P)2BH-group.46 2.3. Catalytic Dehydrogenation of Me2NH-BH3. The potential of complexes 3a−3c as homogeneous catalysts in hydrogenation and dehydrogenation reactions was evaluated for different substrates. Interestingly, all three hydride complexes do not show any significant reactivity toward unsaturated molecules such as olefins and carbonyl compounds in stoichiometric and catalytic experiments, which on the one hand might be a result of the dynamic nature of these complexes. On the other hand, the

2Me2NHBH3 + 2HOt Bu [Fe]

⎯⎯→ (Me2N)2 BH + (t BuO)2 BH + 4H 2

(2)

Utilization of LiN(SiMe3)2 as a base in combination with complex 3b as a catalyst leads to full conversion and similar yields of (Me2N-BH2)2 (82%) like with KOtBu (entry 4). In this reaction HB(NMe2)2 is observed as the major byproduct.

Table 3. Catalytic Dehydrogenation of Me2NH-BH3 Using 3a−3c as Catalysts 2 mol % 3a − 3c,base

2Me2NH‐BH3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ (Me2N‐BH 2)2 + 2H 2 toluene 23° C

entry

precatalyst/basea

1 2 3 4 5c 6 7 8 9 10 11 12

3a/KOtBu 3b/KOtBu 3c/KOtBu 3b/LiN(SiMe3)2 3b/KOtBu 3a/KOtBu 3b/KOtBu 3b/LiN(SiMe3)2 3c/KOtBu 3a/KOtBu 3b/KOtBu 3c/KOtBu

additive

conversion (%)

yieldb (%)

TON

TOF (h−1)

+100 equiv DMAB +Hg drop +Hg drop +Hg drop +Hg drop +0.6 mol % PMe3 +0.6 mol % PMe3 +0.6 mol % PMe3

>99 >99 >99 >99 89 >99 >99 >99 >99 >99 90 95

70 83 62 82 43 73 63 82 58 65 30 45

35 41 31 41 64 36 31 41 29 32 15 22

2.2 2.6 1.9 2.6 1.6 2.2 1.9 2.6 1.8 2.0 0.9 1.4

a

Precatalyst (2 mol %) and base (20 mol %) were suspended in toluene (1 mL) and stirred for 15 min, until a bright orange solution was formed. Me2NH-BH3 (0.54 mmol) dissolved in 1 mL of toluene was added to the catalyst solution, and the mixture was stirred at 23 °C for 16 h. bYield of (Me2N-BH2)2. cSecond addition of substrate and base after a completed dehydrogenation run (S/B/C = 150/30/1). 7303


Research Article

ACS Catalysis

Figure 6. Reactivity of 3b toward base.

Figure 7. Parts of the 13C{1H} NMR spectrum of 5b acquired right after its formation and 13C APT NMR spectrum of the same sample after several days.

PMe3-addition is found for precatalyst 3b, which results in 90% conversion and the formation of (Me2NBH2)2 in 30% yield (entry 11). It should be noted that increasing amounts of PMe3 subsequently limited the catalytic activity. The activity of lowcoordinated iron(0) catalysts is effectively inhibited by chelating diolefins, 7 1 but in the current case dibenzo[a,e]cyclooctatetraene had no impact on the activity. 2.4. Mechanism of the Me2NH-BH3 Dehydrogenation. The poisoning experiments listed in Table 3 had no (3a and 3c) or only little (3b) impact on the catalytic activity of the ironbased catalysts, which at first glance suggests the presence of a homogeneous catalyst for 3a and 3c, while for 3b the reduced yield of (Me2NBH2)2 in the poisoning experiments might be taken as indication for the formation of catalytically active particles. A closer look on the catalytic reactions reveals that this interpretation of the poisoning experiments is questionable. Complexes 3a−3c are colorless and their treatment with a base results in yellow to orange toluene solutions. In the case of precatalysts 3a and 3c the reaction mixture turns dark quickly and results in brownish black, partly magnetic precipitates and decolorized solutions. The 31P{1H} NMR spectra of the supernatant solutions after the catalytic dehydrogenation with 3a and 3c as precatalysts showed no resonances, suggesting that all phosphorus-containing species are either precipitated or paramagnetic. As the employed precatalysts are prone for deprotonation at the NH-function, yielding diamagnetic and neutral complexes in toluene, the decomposition occurs upon treatment of the precatalyst/base mixture with Me2NH-BH3.

For the most active catalyst among this series, 3b (entry 2), we investigated whether the reaction mixture remains catalytically active. Readdition of the substrate Me2NH-BH3(DMAB) after a completed run of catalytic dehydrogenation with KOtBu as a cocatalyst does not result in further catalytic conversion. Readdition of substrate (100 equiv) and KOtBu cocatalyst leads to further catalytic conversion and an overall turnover number (TON) of 64 (entry 5), which is the highest value reported to date for this reaction with an iron-based catalyst. The necessity for base readdition might be rationalized by subsequent base consumption according to eq 2. As previous reports on iron catalysts for the dehydrogenation of Me2NH-BH3 provided evidence for the formation of catalytically active nanoparticles,57,58,67 we performed a number of poisoning experiments to get further insight in the nature of the active species. Addition of mercury to the catalytically active reaction mixture did not result in a reduced activity for 3a and 3c (entries 6 and 9). In the case of complex 3b full conversion of Me2NHBH3 is observed, but the yield of (Me2NBH2)2 is with 63% slightly reduced (entry 7). With LiN(SiMe3)2 as a base no effect on productivity and activity is observed with 3b (entry 8). Treatment of the activated hydrogen liberating catalyst system with 0.6 mol % of PMe3, which in the case of iron nanoparticle formation is expected to inhibit the catalytically active surface of the particle,70 had only a minor impact on the activity of 3a (entry 10). With complex 3c as a catalyst 95% conversion of Me2NH-BH3 is detected and (Me2NBH2)2 is formed in 45% yield (entry 12). The strongest effect of the 7304


Research Article

ACS Catalysis

Figure 8. Free energy profiles for the dehydrogenation of Me2NH-BH3 catalyzed by complex 3b. The free energies are reported in kcal/mol at the M06-L/BSII/SMD(benzene)//M06-L/BSI level of theory.

ligand. 13C APT NMR spectrum shows two resonances corresponding to CH2 groups (δC = 32.1 and 53.3 ppm) as well as only one resonance for the two carbonyl ligands (δC = 221.1 ppm). Overall, these findings suggest that 5b undergoes a slow proton transfer from a rather acidic hydrido ligand to the deprotonated “arm” of the pincer ligand, leading to the iron(0) dicarbonyl complex 6b. This conclusion is further supported by the observation of two new bands for C−O-stretching vibration at lower wavenumbers (ν̃CO = 1833, 1881 cm−1). Upon addition of 10 equiv of KOtBu to 3b a new species is formed that gives rise to two boron bound hydrogen atoms according to the 1H{11B} NMR spectrum (δH = −1.11, 0.34 ppm) and a broad resonance at −50.7 ppm in the 11B{1H} NMR spectrum. The absence of a resonance for a hydrido ligand in the 1 H NMR spectrum in combination with the appearance of two resonances in the 31P{1H} NMR spectrum, a multiplet resonance at 60.4 ppm and a broad resonance at 18.2 ppm, let us speculate that 5b is getting further deprotonated in the presence of excess base. In analogy to 5a, it seems likely that deprotonation of the P-CH2-P moiety in 5b takes place. In the case of the amine-bridged ligand in 3a the deprotonation of the second P-NH-P group led to the stabilization of the iron(0) intermediate46 and the coordination of the resulting diphosphinoborate has been observed for iron(II) complexes, too.72 In the current case the available spectroscopic data gives rise to the assumption that a bisphosphino-borate iron(0) complex [({Ph2P-CH-PPh2}BH2)Fe(CO)2]− (7b) is formed. However, this complex immediately disappears upon addition of DMAB and is not observed during or after the catalytic dehydrogenation reaction. We therefore concluded that it is not relevant for the observed catalytic activity. Addition of substrate (Me2NH-BH3) to mixtures of 3b with different amounts of different bases led to immediate gas evolution and the disappearance of the resonances in the NMR

In contrast, the dehydrogenation reactions with 3b as a precatalyst remain orange and almost clear throughout the entire reaction time. The 1H and 31P{1H} NMR spectra clearly indicate the formation of diamagnetic species, and we therefore decided to investigate systematically the reactivity toward different bases and Me2NH-BH3 (Figure 6). The reaction of 3b with 1 equiv of base (KOtBu, LiN(SiMe3)2 or KH) in C6D6 or toluene-d8 results in the formation of a new iron complex (5b), which gives rise to two broad resonances (δP = 17.8 and 19.9 ppm) as well as a doublet of doublet resonance at 72.6 ppm and a triplet of doublets resonance 98.9 ppm in the 31P{1H} NMR spectrum. The corresponding 11B{1H} NMR spectrum shows a single, broad resonance at −19.4 ppm. The appearance of a broadened triplet resonance at −8.67 ppm in the 1H NMR spectrum clearly indicates the presence of a hydrido ligand. From the comparison of 13C{1H} and 13C attached proton test (APT) NMR data (Figure 7) it becomes evident that one of the CH2 groups in 3b gets deprotonated upon treatment with a base. The remaining CH2 group in 5b gives rise to a doublet of doublets of doublets resonance at 41.2 ppm in the 13C APT NMR spectrum, while the deprotonated CH group leads to a doublet of doublets resonance of opposite phase at 29.1 ppm. Furthermore, two resonances at 216.3 and 218.8 ppm were observed for the two carbonyl ligands, which is in line with two bands 1910 and 1953 cm−1 for the C−O-stretching vibration in the infrared (IR) spectrum. It should be noted that complex 5b is not stable in solution and slowly reacts to a second species (6b). On the basis of the absence of typical resonances for hydrido ligands in the 1H NMR spectrum (even at low temperatures) and the appearance of two resonances in the 31P{1H} NMR spectrum (δP = 26.1 and 75.8 ppm) as well as a new broad resonance at −37.2 ppm in the 11 1 B{ H} NMR spectrum, we concluded that 6b is a more symmetric nonhydride complex with an otherwise intact pincer7305


Research Article

ACS Catalysis

Figure 9. Suggested pathway for the dehydrogenation of Me2NH-BH3.

deprotonation (Figure S43), suggesting an exergonic process of deprotonation. In line with the experimental observations, 6b′, an isomer of complex 6b that differers in the coordination geometry and the coordination mode of the tridentate ligand (fac vs mer) exhibits a difference of only 0.1 kcal/mol in free energy (Figure S43). Therefore, the neutral five coordinated complex 6b as well as 5b are regarded as active species for the dehydrogenation reaction. Moreover, the conversion of complex 5b to 6b assisted by Me2NH-BH3 is feasible, with a calculated energy barrier of 25.1 kcal/mol (TS4 in Figure 8). As shown in Figure 9, the catalytic cycle involves two major steps: (i) the proton transfer and (ii) concerted hydrogen liberation. In the first step, a proton is transferred from Me2NHBH3 to the iron(0) center of 6b via transition state TS1 (ΔG‡ = 24.4 kcal/mol, ΔH‡ = 23.0 kcal/mol), leading to iron(II) intermediate 8b (ΔG = 5.6 kcal/mol, ΔH = 5.6 kcal/mol). The proton transfer process leads to an ion pair, containing the cisisomer of the cationic hexa-coordinated complex 3b with [Me2N-BH3]− as a counterion. The second reaction step may proceed via two pathways. In one pathway, the boron-bound hydride of the amidotrihydridoborate anion [Me2N-BH3]− either interacts with the iron-bound proton of 8b via transition state TS2 (ΔG‡ = 21.3 kcal/mol, ΔH‡ = 21.3 kcal/mol), completing the catalytic cycle by forming H2 and Me2N-BH2. The latter can then further dimerize to give (Me2N-BH2)2. The alternative feasible pathway involves the deprotonation of the CH2-bridge of the coordinated pincer ligand by the [Me2NBH3]-counterion to release hydrogen via TS3. The resulting complex 5b contains a carbanion, which is stabilized by the πaccepting phosphine substituents.73 The carbanion will be involved in the dehydrogenation of a second equivalent of Me2NH-BH3 via the bifunctional transition state TS4 (ΔG‡ = 10.6 kcal/mol), regenerating the active species 6b. Therefore, the nucleophilic iron center in 6b as well as the stable carbanion in 5b should be considered as important reaction sites for the observed catalytic activity (Figure 10). A closer look on the electronic structures of 3b and 6b provides further evidence for the plausible active species. As shown in Figure 11, the second highest occupied molecular orbital (HOMO−1) of complex 3b mainly represents the B → Fe coordination bond, indicating the strong electron donation from the boron atom to the iron center.46 Although the B → Fe bond may be considered as a reactive site as well, the B → Fe promoted dehydrogenation has

spectra corresponding to 5b and 7b. Complex 5b is quickly converted to 6b and another complex (6b′) that gives rise to a very similar set of resonances in the 31P{1H} NMR spectrum (Figure S38d) and an identical resonance like 6b at −37.2 ppm in the 11B{1H} NMR spectrum. On the basis of these findings, we reasoned that 6b′ must be an isomer of 6b. The observations made by NMR and IR spectroscopy suggest that upon deprotonation a mixture of catalytically active complexes is obtained that is converted to two species during the catalytic reaction, which either represent a resting state, an off-cycle intermediate and/or a deactivated complex. Overall, these findings represent a rare example for series of very similar precatalysts, differing only in the substituents at the phosphine donors and the bridging group (CH2 vs NH), that seem to form different types of active catalysts, operationally homogeneous and heterogeneous ones.70 To provide further understanding of the mechanism for the dehydrogenation of Me2NH-BH3 catalyzed by 3b, quantum chemical investigations based on density functional theory (DFT) were performed. On the basis of the experimental results, the most active catalyst, complex 3b, was selected as a catalyst model for DFT calculations. Starting with 3b, various kinds of possible reaction modes for the dehydrogenation of Me2NHBH3, such as a dissociate mode, outer-sphere mode, innersphere mode etc., have been systematically studied (Figure S42). Those reaction modes are derived from a series of possible isomers, including bridging hydride intermediates, intermediates with dissociated donor groups as well as folded intermediates. However, none of these traditional reaction modes can be considered as the plausible mechanism for the dehydrogenation of Me2NH-BH3 due to their high activation free energy (>50 kcal/mol, Figure S42). This is in good agreement with the experimental fact that a base (KOtBu or LiN(SiMe3)2) is required to achieve catalytic activity. Therefore, the true active species should be generated by deprotonation of the cationic complex 3b. The free energies of three possible deprotonation processes suggest that the deprotonation at the Fe−H (to complex 6b) or C−H position (to complex 5b) is more plausible than the deprotonation at the B−H position (to complex 5b′, as shown in Figure S43). The reaction free energy for the deprotonation of 3b with KOtBu to 5b is −18.6 kcal/mol, while the formation of 6b is more favored with a reaction free energy of −22.3 kcal/mol for the 7306


ACS Catalysis

■

Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.9b00882. Experimental details and procedures, spectra, crystallographic details and details of the DFT calculations (PDF) Crystallographic data of the reported complex C48H44BN2P4, Br (CIF) Crystallographic data of the reported complex C19H43BFeN2O3P4, Br (CIF) Crystallographic data of the reported complex C52H46BFeO2P4, 2.5(CH2Cl2), Br (CIF) Crystallographic data of the reported complex C18H44BFeN2O2P4, Br (CIF)

Figure 10. Relevant reaction sites in 6b (left) and 5b (right) for the observed catalytic dehydrogenation of Me2NH-BH3.

to overcome a high activation free energy (51.8 kcal/mol, TSiso6, Figure S42). The MO energy of Fe−H hydride of 3b is calculated to be −8.5 eV, indicating its lower activity. For complex 6b, the B → Fe bond is represented by the HOMO−5 orbital (−4.4 eV) and the HOMO orbital (−3.1 eV) is best described as a distorted d-orbital located with an increased lobe at the vacant coordination site of the square pyramidal iron(0) center. The MO energy indicates that the iron(0) center in 6b is more likely to be the reactive site than the B → Fe bond in complex 3b. The strong donation effect of the boron-based donor enhances the electron density of the iron center, resulting in a sufficiently electron-rich metal center to accept a proton from Me2NH-BH3. These findings are in line with the experimental observation that a base cocatalyst is required to achieve catalytic activity.

■

AUTHOR INFORMATION

Corresponding Authors

*Z. Ke. E-mail: [email protected]. *R. Langer. E-mail: [email protected]. Phone: +49 6421 2825617. Fax: +49 6421 2825653. ORCID

Zhuofeng Ke: 0000-0001-9064-8051 Robert Langer: 0000-0001-5746-9940 Notes

The authors declare no competing financial interest.

■

3. CONCLUSIONS In conclusion, we reported a simple and straightforward route to iron pincer catalysts, containing a central donor group, based on neutral and tricoordinate boron. All complexes undergo a reversible B−H reductive elimination in solution, for which the overall barriers were obtained by an Eyring analysis. The different values for the activation parameters underline the importance of steric and electronic effects on the reactivity of such complexes. Finally, it is demonstrated that this type of iron complexes are competent dehydrogenation catalysts for amine boranes. Although the activity of all precatalysts is similar, the formed active species and the underlying mechanisms are demonstrated to be different: two of the complexes (3a and 3c) give rise to operationally heterogeneous catalysts, while with 3b an operationally homogeneous catalyst is formed. A detailed mechanistic investigation revealed an unusual mechanism, involving an acidic hydrido ligand as a result of the unique B → M binding properties.

ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft (LA 2830/3-2, 2830/5-1 and 2830/6-1), the NSFC (21673301 and 21473261), the Guangdong Natural Science Funds for Distinguished Young Scholar (No. 2015A030306027), the Tip-top Youth Talents of Guangdong special support program (No. 20153100042090537) and the Fundamental Research Funds for the Central Universities.

■

REFERENCES

(1) Maser, L.; Vondung, L.; Langer, R. The ABC in Pincer Chemistry From Amine- to Borylene- and Carbon-Based Pincer-Ligands. Polyhedron 2018, 143, 28−42. (2) Li, Y.; Zhang, J.; Shu, S.; Shao, Y.; Liu, Y.; Ke, Z. Boron-Based Lewis Acid Transition Metal Complexes as Potential Bifunctional Catalysts. Youji Huaxue 2017, 37, 2187. (3) Yamashita, M. Creation of Nucleophilic Boryl Anions and Their Properties. Bull. Chem. Soc. Jpn. 2011, 84, 983−999.

Figure 11. Selected molecular orbitals of complexes 3b (left) and 6b (right). 7307


Research Article

ACS Catalysis

(PPh3)Cl (M = Rh, Ir): Analysis of the Bonding in Metal Borane Compounds. Inorg. Chem. 2006, 45, 2588−2597. (25) Bouhadir, G.; Bourissou, D. Complexes of Ambiphilic Ligands: Reactivity and Catalytic Applications. Chem. Soc. Rev. 2016, 45, 1065− 1079. (26) Bontemps, S.; Sircoglou, M.; Bouhadir, G.; Puschmann, H.; Howard, J. A. K.; Dyer, P. W.; Miqueu, K.; Bourissou, D. Ambiphilic Diphosphine-Borane Ligands: Metal→Borane Interactions within Isoelectronic Complexes of Rhodium, Platinum and Palladium. Chem. - Eur. J. 2008, 14, 731−740. (27) Bontemps, S.; Bouhadir, G.; Gu, W.; Mercy, M.; Chen, C. H.; Foxman, B. M.; Maron, L.; Ozerov, O. V.; Bourissou, D. Metallaboratranes Derived From a Triphosphanyl-Borane: Intrinsic C3 Symmetry Supported by a Z-type Ligand. Angew. Chem., Int. Ed. 2008, 47, 1481−1484. (28) Sircoglou, M.; Bontemps, S.; Mercy, M.; Saffon, N.; Takahashi, M.; Bouhadir, G.; Maron, L.; Bourissou, D. Transition-Metal Complexes Featuring Z-type Ligands: Agreement or Discrepancy Between Geometry and dn Configuration? Angew. Chem., Int. Ed. 2007, 46, 8583−8586. (29) Anderson, J. S.; Rittle, J.; Peters, J. C. Catalytic Conversion of Nitrogen to Ammonia by an Iron Model Complex. Nature 2013, 501, 84−87. (30) Harman, W. H.; Peters, J. C. Reversible H2 Addition Across a Nickel-Borane Unit as a Promising Strategy for Catalysis. J. Am. Chem. Soc. 2012, 134, 5080−5082. (31) Moret, M. E.; Peters, J. C. N2 Functionalization at Iron Metallaboratranes. J. Am. Chem. Soc. 2011, 133, 18118−18121. (32) MacMillan, S. N.; Hill Harman, W.; Peters, J. C. Facile Si−H Bond Activation and Hydrosilylation Catalysis Mediated by a Nickel− Borane Complex. Chem. Sci. 2014, 5, 590−597. (33) Harman, W. H.; Lin, T. P.; Peters, J. C. A d10 Ni-(H2) Adduct as an Intermediate in H-H Oxidative Addition Across a Ni-B Bond. Angew. Chem., Int. Ed. 2014, 53, 1081−1086. (34) Nesbit, M. A.; Suess, D. L.; Peters, J. C. E-H Bond Activations and Hydrosilylation Catalysis with Iron and Cobalt Metalloboranes. Organometallics 2015, 34, 4741−4752. (35) Fong, H.; Moret, M. E.; Lee, Y.; Peters, J. C. Heterolytic H2 Cleavage and Catalytic Hydrogenation by an Iron Metallaboratrane. Organometallics 2013, 32, 3053−3062. (36) Braunschweig, H.; Dewhurst, R. D.; Hupp, F.; Nutz, M.; Radacki, K.; Tate, C. W.; Vargas, A.; Ye, Q. Multiple Complexation of CO and Related Ligands to a Main-Group Element. Nature 2015, 522, 327− 330. (37) Ruiz, D. A.; Melaimi, M.; Bertrand, G. An Efficient Synthetic Route to Stable Bis(carbene)borylenes [(L1)(L2)BH]. Chem. Commun. 2014, 50, 7837−7839. (38) Ruiz, D. A.; Ung, G.; Melaimi, M.; Bertrand, G. Deprotonation of a Borohydride: Synthesis of a Carbene-Stabilized Boryl Anion. Angew. Chem., Int. Ed. 2013, 52, 7590−7592. (39) Kinjo, R.; Donnadieu, B.; Celik, M. A.; Frenking, G.; Bertrand, G. Synthesis and Characterization of a Neutral Tricoordinate Organoboron Isoelectronic with Amines. Science 2011, 333, 610−3. (40) Kong, L.; Ganguly, R.; Li, Y.; Kinjo, R. Diverse Reactivity of a Tricoordinate Organoboron L 2PhB: (L = Oxazol-2-ylidene) Towards Alkali Metal, Group 9 Metal, and Coinage Metal Precursors. Chem. Sci. 2015, 6, 2893−2902. (41) Kong, L.; Li, Y.; Ganguly, R.; Vidovic, D.; Kinjo, R. Isolation of a Bis(oxazol-2-ylidene)-phenylborylene Adduct and its Reactivity as a Boron-Centered Nucleophile. Angew. Chem., Int. Ed. 2014, 53, 9280− 9283. (42) Landmann, J.; Keppner, F.; Hofmann, D. B.; Sprenger, J. A.; Häring, M.; Zottnick, S. H.; Müller-Buschbaum, K.; Ignat’ev, N. V.; Finze, M. Deprotonation of a Hydridoborate Anion. Angew. Chem., Int. Ed. 2017, 56, 2795−2799. (43) Landmann, J.; Sprenger, J. A. P.; Bertermann, R.; Ignat’Ev, N.; Bernhardt-Pitchougina, V.; Bernhardt, E.; Willner, H.; Finze, M. Convenient Access to the Tricyanoborate Dianion B(CN)32− and

(4) Braunschweig, H.; Dewhurst, R. D.; Schneider, A. ElectronPrecise Coordination Modes of Boron-Centered Ligands. Chem. Rev. 2010, 110, 3924−3957. (5) Masuda, Y.; Hasegawa, M.; Yamashita, M.; Nozaki, K.; Ishida, N.; Murakami, M. Oxidative Addition of a Strained C-C Bond onto Electron-Rich Rhodium(I) at Room Temperature. J. Am. Chem. Soc. 2013, 135, 7142−7145. (6) Tanoue, K.; Yamashita, M. Organometallics 2015, 34, 4011. (7) Kwan, E. H.; Kawai, Y. J.; Kamakura, S.; Yamashita, M. A LongTethered (P−B−P)-Pincer Ligand: Synthesis, Complexation, and Application to Catalytic Dehydrogenation of Alkanes. Dalt. Trans. 2016, 45, 15931−15941. (8) Kwan, E. H.; Ogawa, H.; Yamashita, M. ChemCatChem 2017, 9, 2457−2462. (9) Ogawa, H.; Yamashita, M. Platinum Complexes Bearing a BoronBased PBP Pincer Ligand: Synthesis, Structure, and Application as a Catalyst for Hydrosilylation of 1-Decene. Dalton Transactions 2013, 42, 625−629. (10) Owen, G. R. Functional Group Migrations Between Boron and Metal Centres within Transition Metal-Borane and -Boryl Complexes and Cleavage of H-H, E-H and E-E′ Bonds. Chem. Commun. 2016, 52, 10712−10726. (11) Amgoune, A.; Bourissou, D. σ-Acceptor, Z-Type Ligands for Transition Metals. Chem. Commun. 2011, 47, 859−871. (12) Braunschweig, H.; Dewhurst, R. D. Transition Metals as Lewis Bases: “Z-Type” Boron Ligands and Metal-to-Boron Dative Bonding. Dalt. Trans. 2011, 40, 549−558. (13) Hill, A.; Owen, G.; White, A.; Williams, D. The Sting of the Scorpion: A Metallaboratrane. Angew. Chem., Int. Ed. 1999, 38, 2759− 2761. (14) Crossley, I. R.; Hill, A. F.; Willis, A. C. Formation of Metallaboratranes: The Missing Link. The First Iridaboratranes, [IrH(CO)(PPh3){κ3-B,S,S′-B(mt)2R}](Ir→B) (mt = Methimazolyl, R = mt, H). Organometallics 2005, 24, 1062−1064. (15) Crossley, I. R.; Hill, A. F.; Humphrey, E. R.; Smith, M. K. A Less Carbocentric View of Agostic Interactions: The Complexes [Rh(η4cod){H2A(mt)2}] (A = B, C + ; mt = Methimazolyl). Organometallics 2006, 25, 2242−2247. (16) Crossley, I. R.; Hill, A. F. Unlocking the Metallaboratrane Cage: Reversible B-H Activation in Platinaboratranes. Dalt. Trans. 2008, 201−203. (17) Crossley, I. R.; Hill, A. F.; Willis, A. C. Metallaboratranes: Bisand Tris(methimazolyl)borane Complexes of Group 9 Metal Carbonyls and Thiocarbonyls. Organometallics 2010, 29, 326−336. (18) Hill, A. F.; Lee, S. B.; Park, J.; Shang, R.; Willis, A. C. Analogies Between Metallaboratranes, Triboronates, and Boron Pincer Ligand Complexes. Organometallics 2010, 29, 5661−5669. (19) Owen, G. R.; Gould, P. H.; Hamilton, A.; Tsoureas, N. Unexpected Pincer-Type Coordination (κ3 -SBS) Within a Zerovalent Platinum Metallaboratrane Complex. Dalt. Trans. 2010, 39, 49−52. (20) Dyson, G.; Hamilton, A.; Mitchell, B.; Owen, G. R. A New Family of Flexible Scorpionate Ligands Based on 2-Mercaptopyridine. Dalt. Trans. 2009, 93, 6120. (21) Tsoureas, N.; Kuo, Y.-Y.; Haddow, M. F.; Owen, G. R. Double Addition of H2 to Transition Metal-Borane Complexes: a ‘Hydride Shuttle’ Process Between Boron and Transition Metal Centres. Chem. Commun. 2011, 47, 484−486. (22) Pang, K.; Quan, S. M.; Parkin, G. Palladium Complexes with Pd→B Dative Bonds: Analysis of the Bonding in the Palladaboratrane Compound [κ4-B(mim But)3]Pd(PMe3). Chem. Commun. 2006, 37, 5015−5017. (23) Figueroa, J. S.; Melnick, J. G.; Parkin, G. Reactivity of the Metal→ BX3 Dative σ-Bond: 1,2-Addition Reactions of the Fe→BX3 Moiety of the Ferraboratrane Complex [κ4-B(mimBut)3]Fe(CO)2. Inorg. Chem. 2006, 45, 7056−7058. (24) Landry, V. K.; Melnick, J. G.; Buccella, D.; Pang, K.; Ulichny, J. C.; Parkin, G. Synthesis and Structural Characterization of [κ3-B,S,SB(mimR)3]Ir(CO)(PPh3)H(R = But, Ph) and [κ4-B(mimBut)3]M7308


Research Article

ACS Catalysis

Catalysed Dehydrocoupling of Methylamine Borane. Dalt. Trans. 2017, 46, 6843−6847. (63) Lichtenberg, C.; Adelhardt, M.; Gianetti, T. L.; Meyer, K.; De Bruin, B.; Grü tzmacher, H. Low-Valent Iron Mono-Diazadiene Compounds: Electronic Structure and Catalytic Application. ACS Catal. 2015, 5, 6230−6240. (64) Lichtenberg, C.; Viciu, L.; Adelhardt, M.; Sutter, J.; Meyer, K.; De Bruin, B.; Grützmacher, H. Low-Valent Iron(I) Amido Olefin Complexes as Promotors for Dehydrogenation Reactions. Angew. Chem., Int. Ed. 2015, 54, 5766−5771. (65) Lunsford, A. M.; Blank, J. H.; Moncho, S.; Haas, S. C.; Muhammad, S.; Brothers, E. N.; Darensbourg, M. Y.; Bengali, A. A. Catalysis and Mechanism of H2 Release from Amine−Boranes by Diiron Complexes. Inorg. Chem. 2016, 55, 964−973. (66) Coles, N. T.; Mahon, M. F.; Webster, R. L. Phosphine- and Amine-Borane Dehydrocoupling Using a Three-Coordinate Iron(II) ßDiketiminate Precatalyst. Organometallics 2017, 36, 2262−2268. (67) Sonnenberg, J. F.; Morris, R. H. Evidence for Iron Nanoparticles Catalyzing the Rapid Dehydrogenation of Ammonia-Borane. ACS Catal. 2013, 3, 1092−1102. (68) Miller, A. J. M.; Bercaw, J. E. Dehydrogenation of Amine Boranes with a Frustrated Lewis Pair. Chem. Commun. 2010, 46, 1709− 1711. (69) Onak, T.; Landesman, H.; Williams, R.; Shapiro, I. The B11 Nuclear Magnetic Resonance Chemical Shifts and Spin Coupling Values for Various Compounds. J. Phys. Chem. 1959, 63, 1533−1535. (70) Crabtree, R. H. Resolving Heterogeneity Problems and Impurity Artifacts in Operationally Homogeneous Transition Metal Catalysts. Chem. Rev. 2012, 112, 1536−1554. (71) Bü s chelberger, P.; Gä r tner, D.; Reyes-Rodriguez, E.; Kreyenschmidt, F.; Koszinowski, K.; Jacobi von Wangelin, A.; Wolf, R. Alkene Metalates as Hydrogenation Catalysts. Chem. - Eur. J. 2017, 23, 3139−3151. (72) Vondung, L.; Sattler, L. E.; Langer, R. Ambireactive (R3P)2BH2 Groups Facilitating Temperature-Switchable Bond Activation by an Iron Complex. Chem. - Eur. J. 2018, 24, 1358−1364. (73) Hou, C.; Jiang, J.; Li, Y.; Zhao, C.; Ke, Z. When Bifunctional Catalyst Encounters Dual MLC Modes: DFT Study on the Mechanistic Preference in Ru-PNNH Pincer Complex Catalyzed Dehydrogenative Coupling Reaction. ACS Catal. 2017, 7, 786−795.

Selected Reactions as a Boron-Centred Nucleophile. Chem. Commun. 2015, 51, 4989−4992. (44) Bernhardt, E.; Bernhardt-Pitchougina, V.; Willner, H.; Ignatiev, N. Umpolung” at Boron by Reduction of [B(CN)4]− and Formation of the Dianion [B(CN)3]2−. Angew. Chem., Int. Ed. 2011, 50, 12085− 12088. (45) Vondung, L.; Alig, L.; Ballmann, M.; Langer, R. Umpolung at Boron: Ancillary-Ligand-Induced Formation of Boron-Based Donor Ligands from Phosphine-Boranes. Chem. - Eur. J. 2018, 24, 12346− 12353. (46) Vondung, L.; Frank, N.; Fritz, M.; Alig, L.; Langer, R. PhosphineStabilized Borylenes and Boryl Anions as Ligands? Redox Reactivity in Boron-Based Pincer Complexes. Angew. Chem., Int. Ed. 2016, 55, 14450−14454. (47) Frank, N.; Hanau, K.; Flosdorf, K.; Langer, R. Formation of an Iron Phosphine-Borane Complex by Formal Insertion of BH3 into the Fe−P Bond. Dalt. Trans. 2013, 42, 11252. (48) Grätz, M.; Bäcker, A.; Vondung, L.; Maser, L.; Reincke, A.; Langer, R. Donor Ligands Based on Tricoordinate Boron Formed by BH-Activation of Bis(phosphine)boronium Salts. Chem. Commun. 2017, 53, 7230−7233. (49) For further details, please see Supporting Information. (50) Please note that only for the Fe−B bond an arrow is drawn to indicate the L-type nature of the boron-based ligand. In line with conventions in coordination chemistry, all other formal charges and arrows are omitted for clarity. (51) Deegan, M. M.; Peters, J. C. CO Reduction to CH3OSiMe3: Electrophile-Promoted Hydride Migration at a Single Fe Site. J. Am. Chem. Soc. 2017, 139, 2561−2564. (52) Maser, L.; Schneider, C.; Vondung, L.; Alig, L.; Langer, R. Quantifying the Donor Strength of Ligand-Stabilized Main Group Fragments. J. Am. Chem. Soc. 2019, 141, 7596−7604. (53) Maser, L.; Schneider, C.; Alig, L.; Langer, R. Comparing the Acidity of (R3P)2BH-Based Donor Groups in Iridium Pincer Complexes. Inorganics 2019, 7, 61. (54) Morris, R. H. Estimating the Acidity of Transition Metal Hydride and Dihydrogen Complexes by Adding Ligand Acidity Constants. J. Am. Chem. Soc. 2014, 136, 1948−1959. (55) Keaton, R. J.; Blacquiere, J. M.; Baker, R. T. Base Metal Catalyzed Dehydrogenation of Ammonia−Borane for Chemical Hydrogen Storage. J. Am. Chem. Soc. 2007, 129, 1844−1845. (56) Baker, R. T.; Gordon, J. C.; Hamilton, C. W.; Henson, N. J.; Lin, P. H.; Maguire, S.; Murugesu, M.; Scott, B. L.; Smythe, N. C. Iron Complex-Catalyzed Ammonia-Borane Dehydrogenation. A Potential Route Toward B-N-Containing Polymer Motifs using Earth-Abundant Metal Catalysts. J. Am. Chem. Soc. 2012, 134, 5598−5609. (57) Turner, J.; Chilton, N. F.; Kumar, A.; Colebatch, A. L.; Whittell, G. R.; Sparkes, H. A.; Weller, A. S.; Manners, I. Iron Precatalysts with Bulky Tri(tert-butyl)cyclopentadienyl Ligands for the Dehydrocoupling of Dimethylamine-Borane. Chem. - Eur. J. 2018, 24, 14127− 14136. (58) Vance, J. R.; Schäfer, A.; Robertson, A. P. M.; Lee, K.; Turner, J.; Whittell, G. R.; Manners, I. Iron-Catalyzed Dehydrocoupling/ Dehydrogenation of Amine−Boranes. J. Am. Chem. Soc. 2014, 136, 3048−3064. (59) Vance, J. R.; Robertson, A. P.; Lee, K.; Manners, I. Photoactivated, Iron-Catalyzed Dehydrocoupling of Amine-Borane adducts: Formation of Boron-Nitrogen Oligomers and Polymers. Chem. - Eur. J. 2011, 17, 4099−4103. (60) Glüer, A.; Förster, M.; Celinski, V. R.; Schmedt auf der Günne, J.; Holthausen, M. C.; Schneider, S. Highly Active Iron Catalyst for Ammonia Borane Dehydrocoupling at Room Temperature. ACS Catal. 2015, 5, 7214−7217. (61) Bhattacharya, P.; Krause, J. A.; Guan, H. Mechanistic Studies of Ammonia Borane Dehydrogenation Catalyzed by Iron Pincer Complexes. J. Am. Chem. Soc. 2014, 136, 11153−11161. (62) Anke, F.; Han, D.; Klahn, M.; Spannenberg, A.; Beweries, T. Formation of High-Molecular Weight Polyaminoborane by Fe Hydride 7309


Redox-Active, Boron-Based Ligands in Iron Complexes with Inverted

Recommend Documents